The present invention relates to a dynamic pressure bearing used for a rotational machine by which a rotational body can be rotated at high speed. In the dynamic pressure bearing, a recess for generating dynamic pressure is formed between a rotational body and an irrotational body. When the rotational body is rotated, a clearance is formed between the rotational and irrotational bodies by the pneumatic action of the recess for generating dynamic pressure, so that the rotational body can be rotated at high speed through the thus formed clearance.
In general, a rotational body having a dynamic pressure bearing is installed horizontally. By the action of the dynamic pressure bearing, the rotational body is rotated at high speed in the following manner:
A wind generated by high speed rotation of the rotational body is introduced into the recess for generating dynamic pressure formed on the irrotational body. By the action of the wind, a strong wind pressure is generated from the recess. This wind pressure is impressed upon a surface of the rotational body, so that a clearance of about several .mu.m is formed between the surface of the rotational body and the surface of the irrotational body. Since the rotational body is rotated through the clearance, the rotational resistance is reduced. A dynamic pressure bearing is well known, which is used for a polygonal mirror rotated at a high speed of not less than 3000 rpm, wherein a clearance of several .mu.m is formed in the radial and thrust bearings by the action of the recess for generating dynamic pressure. This technique is disclosed in Japanese Utility Model Publication Nos. 38330/1992 and 16574/1993. PA0 A rotational body is supported by radial and thrust bearings of dynamic pressure type. A clearance between the rotational body and the thrust bearing is set to be larger than a clearance between the rotational body and the radial bearing. Even under the condition that the rotational body is arranged being inclined, or under the condition that the rotational body is horizontally arranged, a periphery of the rotational body, the peripheral speed of which is especially high, does not come into contact with the thrust bearing.
When the above dynamic pressure bearing is horizontally installed, a clearance of several .mu.m can be maintained between the rotational and irrotational bodies by the action of a wind generated from the recess for generating dynamic pressure, and the rotational body is rotated at low resistance. In the case where the above dynamic pressure bearing is used for a rotational polygonal mirror for laser beam exposure in a small printer or an image recording apparatus, in some cases, it is impossible to horizontally install the dynamic pressure bearing because the installation space and the arrangement of parts are limited. When the dynamic pressure bearing is installed being inclined together with the rotational polygonal mirror, the clearance of several .mu.m can not be maintained and a portion of the rotational body accidentally comes into contact with an opponent surface. As described above, the polygonal mirror is rotated at high speed not lower than 3000 rpm, and the circumferential speed at the periphery is higher than that at the center. When the rotational body is diagonally arranged as described above, or when vibration is given to the rotational body from the outside of the apparatus, the rotational body can not be supported through the clearance, and a peripheral portion of the rotational body is contacted with a portion of the thrust bearing. The dynamic pressure bearing of the prior art has the above disadvantages.